Gametophyte Generation of the Plant Life Cycle

Gametophyte Development

Gametophytes develop from the germination of spores. Spores are reproductive cells that can give rise to new organisms asexually (without fertilization). They are haploid cells that are produced by meiosis in sporophytes. Upon germination, the haploid spores undergo mitosis to form a multicellular gametophyte structure. The mature haploid gametophyte then produces gametes by mitosis.

This process differs from what is seen in animal organisms. In animal cells, haploid cells (gametes) are only produced by meiosis and only diploid cells undergo mitosis. In plants, the gametophyte phase ends with the formation of a diploid zygote by sexual reproduction. The zygote represents the sporophyte phase, which consists of the plant generation with diploid cells. The cycle begins anew when the diploid sporophyte cells undergo meiosis to produce haploid spores.

Gametophyte Generation in Non-vascular Plants

The gametophyte phase is the primary phase in non-vascular plants, such as mosses and liverworts. Most plants are heteromorphic, meaning that they produce two different types of gametophytes. One gametophyte produces eggs, while the other produces sperm. Mosses and liverworts are also heterosporous, meaning that they produce two different types of spores. These spores develop into two distinct types of gametophytes; one type produces sperm and the other produces eggs. The male gametophyte develops reproductive organs called antheridia (produce sperm) and the female gametophyte develops archegonia (produce eggs).

Non-vascular plants must live in moist habitats and rely on water to bring the male and female gametes together. Upon fertilization, the resulting zygote matures and develops into a sporophyte, which remains attached to the gametophyte. The sporophyte structure is dependent upon the gametophyte of nourishment because only the gametophyte is capable of photosynthesis. The gametophyte generation in these organisms consists of the green, leafy or moss-like vegetation located at the base of the plant. The sporophyte generation is represented by the elongated stalks with spore-containing structures at the tip.

Gametophyte Generation in Vascular Plants

In plants with vascular tissue systems, the sporophyte phase is the primary phase of the life cycle. Unlike in non-vascular plants, the gametophyte and sporophyte phases in non-seed producing vascular plants are independent. Both the gametophyte and the sporophyte generations are capable of photosynthesis. Ferns are examples of these types of plants. Many ferns and other vascular plants are homosporous, meaning that they produce one type of spore. The diploid sporophyte produces haploid spores (by meiosis) in specialized sacs called sporangia.

Sporangia are found on the underside of the fern leaves and release spores into the environment. When a haploid spore germinates, it divides by mitosis forming a haploid gametophyte plant called a prothallium. The prothallium produces both male and female reproductive organs, which form sperm and eggs respectively. Water is needed for fertilization to take place as sperm swim toward the female reproductive organs (archegonia) and unite with the eggs. After fertilization, the diploid zygote develops into a mature sporophyte plant that arises from the gametophyte. In ferns, the sporophyte phase consists of the leafy fronds, sporangia, roots, and vascular tissue. The gametophyte phase consists of the small, heart-shaped plants or prothallia.

Gametophyte Generation in Seed Producing Plants

In seed producing plants, such as angiosperms and gymnosperms, the microscopic gametophyte generation is totally dependent upon the sporophyte generation. In flowering plants, the sporophyte generation produces both male and female spores. Male microspores (sperm) form in microsporangia (pollen sacs) in the flower stamen. Female megaspores (eggs) form in megasporangium in the flower ovary. Many angiosperms have flowers that contain both microsporangium and megasporangium.

The fertilization process occurs when pollen is transfered by wind, insects, or other plant pollinators to the female portion of the flower (carpel). The pollen grain germinates forming a pollen tube that extends downward to penetrate the ovary and allow a sperm cell to fertilize the egg. The fertilized egg develops into a seed, which is the beginning of a new sporophyte generation. The female gametophyte generation consists of the megaspores with embryo sac. The male gametophyte generation consists of microspores and pollen. The sporophyte generation consists of the plant body and seeds.

Gametophyte Key Takeaways

• The plant life cycle alternates between a gametophyte phase and a sporophyte phase in a cycle known as alternation of generations.
• The gametophyte represents the sexual phase of the life cycle as gametes are produced in this phase.
• Plant sporophytes represent the asexual phase of the cycle and produce spores.
• Gamatophytes are haploid and develop from spores generated by sporophytes.
• Male gametophytes produce reproductive structures called antheridia, while female gametophytes produce archegonia.
• Non-vascular plants, like mosses and liverworts, spend most of their life cycle in the gametophyte generation.
• The gametophye in non-vascular plants is the green, moss-like vegetation at the base of the plant.
• In seedless vascular plants, such as ferns, the gametophyte and sporophyte generations are both capable of photosynthesis and are independent.
• The gametophyte structure of ferns is a heart-shaped plant called a prothallium.
• In seed-bearing vascular plants, such as angiosperms and gymnosperms, the gametophyte is totally dependent on the sporophyte for development.
• Gametophytes in angiosperms and gymnosperms are pollen grains and ovules.

Sources

• _{Gilbert, Scott F. “Plant Life Cycles.” Developmental Biology. 6th Edition., U.S. National Library of Medicine, 1 Jan. 1970, www.ncbi.nlm.nih.gov/books/NBK9980/.}
• _{Graham, L K, and L W Wilcox. “The Origin of Alternation of Generations in Land Plants: a Focus on Matrotrophy and Hexose Transport.” Philosophical Transactions of the Royal Society B: Biological Sciences, U.S. National Library of Medicine, 29 June 2000, www.ncbi.nlm.nih.gov/pmc/articles/PMC1692790/.}